CHAPTER 2 ALKANES SOLUTIONS TO TEXT PROBLEMS 2.1 A carbonyl group is C=O. Of the two carbonyl functions in prostaglandin E, one belongs to the ketone family, the other to the carboxylic acids functional OH 2.2 An unbranched alkane(n-alkane)of 28 carbons has 26 methylene(CH,) groups flanked by a methyl (CH3) group at each end. The condensed formula is CH3(CH,)26CH3. 2.3 The alkane represented by the carbon skeleton formula has ll carbons. The general formula for an alkane is CnH,n+2, and thus there are 24 hydrogens. The molecular formula is ClH 4; the condensed structural formula is CH3(CH)CH 2.4 In addition to CH3 (CH2)4CH3 and(CH,)2CHCH,CHa CH3, there are three more isomers. One has a five-carbon chain with a one-carbon(methyl) branch CHa CH, CHCH, CH The remaining two isomers have two methyl branches on a four-carbon chain. H3 CH3 CHCHCH3 CH, CH, CCH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
25 CHAPTER 2 ALKANES SOLUTIONS TO TEXT PROBLEMS 2.1 A carbonyl group is C?O. Of the two carbonyl functions in prostaglandin E1 one belongs to the ketone family, the other to the carboxylic acids. 2.2 An unbranched alkane (n-alkane) of 28 carbons has 26 methylene (CH2) groups flanked by a methyl (CH3) group at each end. The condensed formula is CH3(CH2)26CH3. 2.3 The alkane represented by the carbon skeleton formula has 11 carbons. The general formula for an alkane is CnH2n2, and thus there are 24 hydrogens. The molecular formula is C11H24; the condensed structural formula is CH3(CH2)9CH3. 2.4 In addition to CH3(CH2)4CH3 and (CH3)2CHCH2CH2CH3, there are three more isomers. One has a five-carbon chain with a one-carbon (methyl) branch: The remaining two isomers have two methyl branches on a four-carbon chain. CH or or 3CHCHCH3 CH3 CH3 CH3CH2CCH3 CH3 CH3 CH3CH2CHCH2CH3 CH3 or Ketone OH functional group Carboxylic acid functional group O O HO OH Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
26 ALKANES 2.5 (b) Octacosane is not listed in Table 2.4, but its structure can be deduced from its systematic name. The suffix-cosane pertains to alkanes that contain 20-29 carbons in their longest con- nuous chain. The prefix octa- means"eight. Octacosane is therefore the unbranched alkane having 28 carbon atoms. It is CHi(CH))2 CH (c) The alkane has an unbranched chain of ll carbon atoms and is named undecane 2.6 The ending -hexadecane reveals that the longest continuous carbon chain has 16 carbon atoms There are four methyl groups(represented by tetramethyl-), and they are located at carbons 2, 6, 10, and 14 2, 6, 10, 14-Tetramethylhexadecane (phytane) 2.7 (b) The systematic name of the unbranched CsHiz isomer is pentane(Table 2. 4) CHaCH,CH,CH,CH A second isomer, (CH3)2 CHCH,CH3, has four carbons in the longest continuous chain and so is named as a derivative of butane. Since it has a methyl group at C-2, it is 2-methylbutane IUPAC name: 2-methylbutane Common name: isopentane methyl group at C-2 The remaining isomer, (CH3)4C, has three carbons in its longest continuous chain and so is named as a derivative of propane. There are two methyl groups at C-2, and so it is a 2, 2-dimethyl derivative of propane CH CCH IUPAC name: 2, 2-dimethylpropane Common name: neopenta (c) First write out the structure in more detail, and identify the longest continuous carbon chain CH CH3C—CH2-CCH3 There are five carbon atoms in the longest chain, and so the compound is named as a deriva tive of pentane. This five-carbon chain has three methyl substituents attached to it, making it Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
26 ALKANES 2.5 (b) Octacosane is not listed in Table 2.4, but its structure can be deduced from its systematic name. The suffix -cosane pertains to alkanes that contain 20–29 carbons in their longest continuous chain. The prefix octa- means “eight.” Octacosane is therefore the unbranched alkane having 28 carbon atoms. It is CH3(CH2)26CH3. (c) The alkane has an unbranched chain of 11 carbon atoms and is named undecane. 2.6 The ending -hexadecane reveals that the longest continuous carbon chain has 16 carbon atoms. There are four methyl groups (represented by tetramethyl-), and they are located at carbons 2, 6, 10, and 14. 2.7 (b) The systematic name of the unbranched C5H12 isomer is pentane (Table 2.4). A second isomer, (CH3)2CHCH2CH3, has four carbons in the longest continuous chain and so is named as a derivative of butane. Since it has a methyl group at C-2, it is 2-methylbutane. The remaining isomer, (CH3)4C, has three carbons in its longest continuous chain and so is named as a derivative of propane. There are two methyl groups at C-2, and so it is a 2,2-dimethyl derivative of propane. (c) First write out the structure in more detail, and identify the longest continuous carbon chain. There are five carbon atoms in the longest chain, and so the compound is named as a derivative of pentane. This five-carbon chain has three methyl substituents attached to it, making it CH3 C CH3 CH3 CH2 C H CH3 CH3 IUPAC name: 2,2-dimethylpropane Common name: neopentane CH3CCH3 CH3 CH3 IUPAC name: 2-methylbutane Common name: isopentane methyl group at C-2 CH3CHCH2CH3 CH3 IUPAC name: pentane Common name: n-pentane CH3CH2CH2CH2CH3 2,6,10,14-Tetramethylhexadecane (phytane) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALKANES a trimethyl derivative of pentane. Number the chain in the direction that gives the lowest numbers to the substituents at the first point of difference C H CH H H - C-CH CH3C-CH2-C—CH3 2, 2, 4-Trimethylpentane(correct) 2.4, 4-Trimethylpentane(incorrect) (d) The longest continuous chain in(CH3)CC(CH3)3 contains four carbon atoms. CHCH CH -CH The compound is named as a tetramethyl derivative of butane; it is 2, 2, 3, 3-tetramethylbutane. 2.8 There are three CsHu alkyl groups with unbranched carbon chains. One is primary, and two are sec- ondary. The IUPAC name of each group is given beneath the structure. Remember to number the alkyl groups from the point of attachment. CH3,,,CH CHaCH,CHLCHCH CH-CH, CHCH, CH3 Pentyl group(primary) 1-Methylbutyl group(secondary) 1-Ethylpropyl group(secondary) Four alkyl groups are derived from(CH3)2CHCH, CH. Two are primary, one is secondary, and one IS CH H.CH CHCh,CH 3-Methylbutyl group(primary) 2-Methylbutyl group(prim CHa CCH2CH CH, CHCHCH 1, 1-Dimethylpropyl group(tert 1. 2-Dimethylpropyl group(secondary) 2.9(b) Begin by writing the structure in more detail, showing each of the groups written in parenthe ses. The compound is named as a derivative of hexane, because it has six carbons in its longest continuous chain CH, CH, CHCH,CHCH CH3CH2 CH The chain is numbered so as to give the lowest number to the substituent that appears closest to the end of the chain in this case it is numbered so that the substituents are located at C-2 and C-4 rather than at C-3 and C-5. In alphabetical order the groups are ethyl and methyl; they are listed in alphabetical order in the name. The compound is 4-ethyl-2-methy lhexane. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
a trimethyl derivative of pentane. Number the chain in the direction that gives the lowest numbers to the substituents at the first point of difference. (d) The longest continuous chain in (CH3)3CC(CH3)3 contains four carbon atoms. The compound is named as a tetramethyl derivative of butane; it is 2,2,3,3-tetramethylbutane. 2.8 There are three C5H11 alkyl groups with unbranched carbon chains. One is primary, and two are secondary. The IUPAC name of each group is given beneath the structure. Remember to number the alkyl groups from the point of attachment. Four alkyl groups are derived from (CH3)2CHCH2CH3. Two are primary, one is secondary, and one is tertiary. 2.9 (b) Begin by writing the structure in more detail, showing each of the groups written in parentheses. The compound is named as a derivative of hexane, because it has six carbons in its longest continuous chain. The chain is numbered so as to give the lowest number to the substituent that appears closest to the end of the chain. In this case it is numbered so that the substituents are located at C-2 and C-4 rather than at C-3 and C-5. In alphabetical order the groups are ethyl and methyl; they are listed in alphabetical order in the name. The compound is 4-ethyl-2-methylhexane. CH3CH2CHCH2CHCH3 CH3CH2 CH3 6 5 43 21 2-Methylbutyl group (primary) 1,2-Dimethylpropyl group (secondary) 3-Methylbutyl group (primary) CH3CHCH2CH2 4 3 2 1 CH3 CH2CHCH2CH3 1 2 3 4 CH3 CH3CHCHCH3 3 1 2 CH3 1,1-Dimethylpropyl group (tertiary) CH3CCH2CH3 1 2 3 CH3 CH3CH2CH2CH2CH2 Pentyl group (primary) CH3CH2CH2CHCH3 4 3 2 1 1-Methylbutyl group (secondary) CH3CH2CHCH2CH3 3 2 1 1-Ethylpropyl group (secondary) CH3 C CH3 CH3 CH3 C CH3 CH3 CH C not 3 CH3 CH3 CH2 C H CH3 CH3 1 2 45 3 2,2,4-Trimethylpentane (correct) CH3C CH3 CH3 CH2 C H CH3 CH3 54 2 1 3 2,4,4-Trimethylpentane (incorrect) ALKANES 27 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
28 ALKANES (c) The longest continuous chain is shown in the structure; it contains ten carbon atoms. The structure also shows the numbering scheme that gives the lowest number to the substituent at the first point of difference CH CH, CH, CHCH, CHCH, CHCHCH CH,CH3 CH,CHCH In alphabetical order, the substituents are ethyl(at C-8), isopropyl at(C-4), and two methyl groups(at C-2 and C-6). The alkane is 8-ethyl-4-isopropyl-2, 6-dimethyldecane. The system- tic name for the isopropyl group(1-methylethyl) may also be used, and the name becomes 8-ethy l-2, 6-dimethyl-4-(1-methylethyl)decane 2.10 (b) There are ten carbon atoms in the ring in this cycloalkane, thus it is named as a derivative of cyclodecane HC、CH3 (CH3),CH Cvclodecane The numbering pattern of the ring is chosen so as to give the lowest number to the substituent at the first point of difference between them. Thus, the carbon bearing two methyl groups is C-1, and the ring is numbered counterclockwise, placing the isopropyl group on C-4 (numbering clockwise would place the isopropyl on C-8) Listing the substituent groups in al phabetical order, the correct name is 4-isopropyl-1, l-dimethylcyclodecane. Alternatively, the ystematic name for isopropyl(1-methylethyl) could be used, and the name would become (c) When two cycloalkyl groups are attached by a single bond, the compound is named as a cycloalkyl-substituted cycloalkane. This compound is cyclohexylcyclohexane 2.11 The alkane that has the most carbons(nonane) has the highest boiling point (151C). Among the others, all of which have eight carbons, the unbranched isomer(octane) has the highest boiling point (126.C)and the most branched one(2, 2, 3, 3-tetramethylbutane)the lowest(106.). The remaining kane, 2-methy heptane, boils at 116.C 2.12 All hydrocarbons burn in air to give carbon dioxide and water. To balance the equation for the com- oustion of cyclohexane( CHv), first balance the carbons and the hydrogens on the right side. Then balance the oxygens on the left side +90 6CO t 6H,O Cyclohexane Oxygen Carbon dioxide Water 2. 13 (b) Icosane(Table 2. 4)is Cz Ha2. It has four more methylene( CH, ) groups than hexadecane, the last Unbranched alkane in Table 2.5. Its calculated heat of combustion is therefore (4 X 653 kJ/mol) Heat of combustion of icosane= heat of combustion of hexadecane+ 4 x653 kJ/mol = 10.701 kJ/mol 2612 kJ/mol 13. 313 kJ/mol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) The longest continuous chain is shown in the structure; it contains ten carbon atoms. The structure also shows the numbering scheme that gives the lowest number to the substituent at the first point of difference. In alphabetical order, the substituents are ethyl (at C-8), isopropyl at (C-4), and two methyl groups (at C-2 and C-6). The alkane is 8-ethyl-4-isopropyl-2,6-dimethyldecane. The systematic name for the isopropyl group (1-methylethyl) may also be used, and the name becomes 8-ethyl-2,6-dimethyl-4-(1-methylethyl)decane. 2.10 (b) There are ten carbon atoms in the ring in this cycloalkane, thus it is named as a derivative of cyclodecane. The numbering pattern of the ring is chosen so as to give the lowest number to the substituent at the first point of difference between them. Thus, the carbon bearing two methyl groups is C-1, and the ring is numbered counterclockwise, placing the isopropyl group on C-4 (numbering clockwise would place the isopropyl on C-8). Listing the substituent groups in alphabetical order, the correct name is 4-isopropyl-1,1-dimethylcyclodecane. Alternatively, the systematic name for isopropyl (1-methylethyl) could be used, and the name would become 1,1-dimethyl-4-(1-methylethyl)cyclodecane. (c) When two cycloalkyl groups are attached by a single bond, the compound is named as a cycloalkyl-substituted cycloalkane. This compound is cyclohexylcyclohexane. 2.11 The alkane that has the most carbons (nonane) has the highest boiling point (151°C). Among the others, all of which have eight carbons, the unbranched isomer (octane) has the highest boiling point (126°C) and the most branched one (2,2,3,3-tetramethylbutane) the lowest (106°C). The remaining alkane, 2-methylheptane, boils at 116°C. 2.12 All hydrocarbons burn in air to give carbon dioxide and water. To balance the equation for the combustion of cyclohexane (C6H12), first balance the carbons and the hydrogens on the right side. Then balance the oxygens on the left side. 2.13 (b) Icosane (Table 2.4) is C20H42. It has four more methylene (CH2) groups than hexadecane, the last unbranched alkane in Table 2.5. Its calculated heat of combustion is therefore (4 653 kJ/mol) higher. Heat of combustion of icosane heat of combustion of hexadecane 4 653 kJ/mol 10,701 kJ/mol 2612 kJ/mol 13,313 kJ/mol 9O2 6CO2 6H2O Cyclohexane Oxygen Carbon dioxide Water Cyclodecane (CH3)2CH H3C CH3 3 6 7 2 1 8 5 4 9 10 CH3CH2CHCH2CHCH2CHCHCH3 CH2CH3 CH2CHCH3 CH3 CH3 CH3 3 2 1 10 9 87 6 4 5 28 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALKANES 2.14 Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1)the number of carbon atoms and(2) the extent of chain branching. Pentane, isopentane, and neopentane are all CsH12; hexane is CHia. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion; neopentane is the most branched and has the lowest heat of Hexane CHa(CH))4CH3 Heat of combustion 4163 kJ/mol (995.0 kcal/mol) Pentane CH3 CH,CH,CH,CH3 Heat of combustion 3527 kJ/mol (845.3 kcal/mol) (CH3),CHCH,CH Heat of combustion 3529 kJ/mol (843. 4 kcal/mol) Neopentane (CH,)4C Heat of combustion 3514 kJ/mol (839.9 kcal/mol 2.15(b)In the reaction CH,=CH,+ Br2 BrCH,CH, Bi carbon becomes bonded to an atom(Br) that is more electronegative than itself. Carbon is (c) In the reaction 6CH,=CH,+ B,H6 2(CHCH,)3B one carbon becomes bonded to hydrogen and is, therefore, reduced. The other carbon is also reduced, because it becomes bonded to boron, which is less electronegative than carbon. 2.16 It is best to approach problems of this type systematically. Since the problem requires all the isomers of ChI to be written, begin with the unbranched isomer heptane CHCH,CH,CH,CH,CH,CH Two isomers have six carbons in their longest continuous chain. One bears a methyl substituent at C-2, the other a methyl substituent at C-3 (CH3) CHCH, CH- CH,CH3 CH CH CHCH,- CH3 H3 2-Methylhexane 3-Methylhexane Now consider all the isomers that have two methyl groups as substituents on a five-carbon continu- (CH3)3CCH, CHCH3 (CH;CH,)2 C(CH3) 3, 3-Dimethylpentane CH3)CHCHCH,CH (CH3)2 CHCH,CH(CH3h2 4-Dimethylpentane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
2.14 Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1) the number of carbon atoms and (2) the extent of chain branching. Pentane, isopentane, and neopentane are all C5H12; hexane is C6H14. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion; neopentane is the most branched and has the lowest heat of combustion. Hexane CH3(CH2)4CH3 Heat of combustion 4163 kJ/mol (995.0 kcal/mol) Pentane CH3CH2CH2CH2CH3 Heat of combustion 3527 kJ/mol (845.3 kcal/mol) Isopentane (CH3)2CHCH2CH3 Heat of combustion 3529 kJ/mol (843.4 kcal/mol) Neopentane (CH3)4C Heat of combustion 3514 kJ/mol (839.9 kcal/mol) 2.15 (b) In the reaction carbon becomes bonded to an atom (Br) that is more electronegative than itself. Carbon is oxidized. (c) In the reaction one carbon becomes bonded to hydrogen and is, therefore, reduced. The other carbon is also reduced, because it becomes bonded to boron, which is less electronegative than carbon. 2.16 It is best to approach problems of this type systematically. Since the problem requires all the isomers of C7H16 to be written, begin with the unbranched isomer heptane. Two isomers have six carbons in their longest continuous chain. One bears a methyl substituent at C-2, the other a methyl substituent at C-3. Now consider all the isomers that have two methyl groups as substituents on a five-carbon continuous chain. 2,3-Dimethylpentane (CH3)2CHCHCH2CH3 CH3 2,4-Dimethylpentane (CH3)2CHCH2CH(CH3)2 2,2-Dimethylpentane (CH3)3CCH2CH2CH3 3,3-Dimethylpentane (CH3CH2)2C(CH3)2 2-Methylhexane (CH3)2CHCH2CH2CH2CH3 3-Methylhexane CH3CH2CHCH2CH2CH3 CH3 Heptane CH3CH2CH2CH2CH2CH2CH3 B2H6 2(CH3CH2) 6CH2 CH2 3B Br BrCH2CH2Br CH2 CH2 2 ALKANES 29 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website